In recent years, batteries such as lithium ion secondary batteries are used in various fields, for example, electronic devices such as a mobile phone and a personal computer, vehicles such as a hybrid vehicle and an electric vehicle. In particular, the lithium ion secondary battery has high energy density and thus is suitable for various devices.
For example, the lithium ion secondary battery is configured as below. The lithium ion secondary battery includes a power generating element of a flattened shape formed of a positive electrode foil applied with positive active material, a negative electrode foil applied with negative active material, and separators insulating them, which are wound together. The power generating element is enclosed in a case body member having an opening. This opening is closed by a case lid member welded to the case body member. To the power generating element, an electrode terminal member is electrically connected. The electrode terminal member includes for example an element connecting terminal electrically connected to the power generating element and extended out by penetrating through the case lid member, and an outer connecting terminal electrically connected to the element connecting terminal outside the case lid member. The case lid member is provided with a liquid inlet through which an electrolyte is poured. After pouring the electrolyte, the liquid inlet is closed with a plug. This plug is sealingly welded to the case lid member by laser welding.
Herein, a sealing member (a gasket) made of synthetic resin is provided between the element connecting terminal of the electrode terminal member and the case lid member to seal between them. As a battery using a gasket to seal between the element connecting terminal of the electrode terminal member and the case lid member, there is known a lithium ion secondary battery disclosed for example in Patent Document 1 listed below. In the secondary battery disclosed in Patent Document 1 includes, as shown in FIGS. 4 and 5 and paragraph 0017, an element connecting terminal (a positive current collector 14 and a positive connecting terminal 15 in Patent Document 1) includes an insert-through part (a shaft part 15a in Patent Document 1) inserted in a through hole formed through the case lid member (a lid 11 in Patent Document 1) to protrude out of the case lid member and a flat plate part (a head part 15f in Patent Document 1) having a larger diameter than the insert-through part. The element connecting terminal is inserted in the through hole of the case lid member while the gasket is fitted in the insert-through part. Specifically, the gasket is formed therethrough with an insert-through hole (a through hole 10a in Patent Document 1). The element connecting terminal is placed so that the insert-through part is inserted in the insert-through hole and extends through the through hole of the case lid member from a lower surface side of the case lid member. Furthermore, an outer insulating member (an insulating member 12 in Patent Document 1) and an outer connecting terminal (a positive outer terminal 13 in Patent Document 1) are fitted on the insert-through part exposed out of the case lid member and also a tip end of the insert-through part is deformed or riveted by rotary caulking to thereby fasten each member to the case lid member. At that time, the gasket is pressed against the inner surface of the case lid member by the flat plate part. Accordingly, the power connecting terminal and the gasket are placed in surface contact without gaps and the case lid member and the gasket are placed in surface contact without gaps. This ensures sealing between the power connecting terminal and the case lid member and thus can prevent leakage of the electrolyte from the inside of the case body member to the outside of the case lid member.